Originally Posted by Major Nelson
My street Vega has a SBC 427 9.88:1 compression stroker motor, TH400 w/3000 stall converter and a 12 bolt w/3:73 posi.
A chassis dyno was performed and the results were both disappointing and impressive. Seven runs were done. At 5500 RPM's, the engine HP is 357 and the torque is 549. I'm good with the monster torque but I was expecting at least one HP per cubic inch. Two of the runs are on You Tube under '72 Chevrolet Vega dyno run.
On the engine build, I opted to re-use the Edelbrock "Performer RPM" 64cc heads and the dual plane "Performer RPM/Air Gap" intake from the old motor; both are ported and polished. How much HP did I give up by not using better flowing heads? How much by not using for example a single plane intake?
Are those dyno #'s respectable considering the car? My intentions are for street use; I don't have any plans to race it.
Thanks in advance for your replies.
Aside to comparisons to other chassis horsepower numbers I'm of the opinion that they don't tell much about crankshaft horsepower.
In my experience, rear wheel hp measures are about 43% low to SAE Gross power. SAE Gross test would be the set up used prior to 1972 by the OEMs. This test configuration has the minimum accessories on the engine and them turning but in the case of things like the alternator, not charging so all that would include is bearing, brush, and fan drag. The engine would probably have headers and no exhaust system or air cleaner.
The purveyors of chassis dynos like to use a 20 percent expansion difference between the wheels and the crankshaft which is conservative especially in your case with the power hungry TH400 and a 12 bolt, posi rear end.
So if I take the 357 number which is adjusted up 20% from a raw reading of 298 times my factor of 1.43 for my Gross power on crank adjustment this would get you 425. When comparing your cam, compression, and displacement, such a power number compares pretty favorably to the advertised numbers of 427 engines from the muscle car era. While the installed engines tended to perform better than this so people accused the factoriy's of low balling, the test actuyally were real, but with a cam of about 280 degrees and .45 inch lift. Where the as bought engine often had cams over 300 degrees of duration and .5 inch lift, so they didn't actually lie, you had to read the specs against the numbers to figure out where things really were.
Now these days the OEMs use the SAE Net test which has the engine pretty much in the configuration it would have as installed in the car. Roughly these numbers are about 50-70 horses less that the numbers you see on the SAE Gross test. So 425 minus 50 gets 375 and minus 70 gets 355 which has your 357 number bounded.